1. Field of the Invention
This invention relates to a solid-state image sensing device and, for example, a CMOS image sensor comprising a vertical shift register that realizes “sub-sampling”, “averaging” of pixels (signal charges) in a vertical direction of an image sensing region or “cutting out” of pixels in a desired range.
2. Description of the Related Art
A CMOS image sensor is used in a portable telephone with an image sensor, digital camera or video camera. Recent CMOS image sensors have 5,000,000 or more pixels. In a still image photographing operation, photographed images (video signal) with high resolution can be attained by reading all of the pixels. However, the frame rate in the “whole pixel reading operation”, for example, the frame rate in the read operation of whole 5,000,000 pixels is approximately 15 frames/sec. The value is not sufficiently large as the rate of a “viewfinder mode” in a digital still camera or “motion image photographing mode” in a video camera in which a high frame rate is required, for example.
As a method for enhancing the frame rate, a method for compressing photographed images is generally known (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2008-172608). The operation of compressing photographed images is generally performed by “sub-sampling”. With the method, the photographed image is compressed by not reading some pixels in row and column directions of the image sensing region of the CMOS image sensor. The resolution is lowered by “sub-sampling”, but a sufficiently high frame rate can be attained by “motion image photographing”. However, in a simple “sub-sampling” process in a single-plate image sensor, since the distance between pixels becomes larger, false color occurs and the image quality is degraded.
As the measure against the degradation in the image quality due to occurrence of false color, a method of “pixel averaging” is known (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2007-173950). Like the “sub-sampling” method, the above method is a method for compressing photographed images, but a method for averaging luminance values of a plurality of pixels in a vertical or horizontal direction containing sub-sampled pixels without casting the sub-sampled pixels unlike the “sub-sampling” method. Since the luminance values of the sub-sampled pixels are also reflected, false color is difficult to occur.
Further, it is important that the CMOS image sensor satisfies a video signal standard of a Standard Definition (SD) system or High Definition TV (HDTV) system. This is because it becomes disadvantageous in cost and structure if image sensors having specifications corresponding to the respective systems are mounted on a camera, and therefore, it becomes necessary to switch and use one of the sensors. In order to cope with the plural specifications, a method for arranging pixels to satisfy the respective video signal standards (systems) and adjusting the size of a photographed image by “cutting out” is considered (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2005-184358).
However, in the prior art technique, it is necessary to provide extra logic gates in drive circuits of respective rows for the countermeasure against blooming of pixels that are sub-sampled and are not read in the case of Jpn. Pat. Appln. KOKAI Publication No. 2008-172608 in which a “sub-sampling” process is realized. Therefore, the technique is disadvantageous in a circuit area.
In Jpn. Pat. Appln. KOKAI Publication No. 2007-173950 in which “pixel averaging” is proposed, pixels are averaged by means of resistors by using the output impedance of a source-follower circuit. Therefore, it is not necessary to additionally provide circuit elements such as capacitors and “pixel averaging” can be attained by means of a small-scale circuit. However, in Jpn. Pat. Appln. KOKAI Publication No. 2007-173950, the image sensing region is divided into blocks of every four rows and the number of to-be-averaged pixels is changed according to four select signals supplied to each block. Therefore, the degree of freedom is suppressed and, for example, the combination of to-be-averaged pixels is limited within the block or the select circuit is made larger in proportion to the number of pixels in order to increase the number of to-be-averaged pixels.
In Jpn. Pat. Appln. KOKAI Publication No. 2005-184358 in which “cutting out” is proposed, there is provided a logic gate that supplies a control pulse for discharging signal charges to a row that is not read at the “cutting out” time. Therefore, when it becomes necessary to cope with different video signal standards, logic gates are required for respective standards and the circuit area increases accordingly.